Rendering glitches in Microsoft's flagship MSN.com site have sparked renewed criticism from a competitor that the software giant is undermining its browser.

Opera Software on Wednesday said Microsoft has been sending its browser a faulty style sheet, which determines the presentation of graphics and text in a browser window. When people using Opera 7 browser software visit MSN.com, published by Microsoft, some of the site content is obscured, Opera Chief Technology Officer Hakon Lie wrote in a posting to the company's Web site.

"Opera 7 receives a style sheet which is very different from (that used by) the Microsoft and Netscape browsers," Lie wrote in his explanation. "Opera 7 is explicitly instructed to move content off the side of its container, thus creating the impression that there is something wrong with Opera 7."

Not only does the code not work in Opera 7, it doesn't work in Microsoft's own Internet Explorer 6 either, CNET News.com confirmed.

After an initial denial, Microsoft on Wednesday said it did send different style sheets to different browsers, and defended the practice.

"We have different style sheets and different code for various browsers," said Bob Visse, director of marketing for MSN. "That's something we do to try optimize the experience for our users."

When asked why Opera was sent a different style sheet, when the IE style sheet would render properly in Opera, Visse said only that the Opera style sheet was the same one that MSN sent to other, earlier versions of IE.

"There's no reason we wouldn't want to have the best experience for all the browsers out there, so we're certainly going to look into that," he said.

The browser-breaking charges come more than a year after Microsoft fell under fire for blocking Opera and other non-IE browsers from access to its MSN site.

In the resulting outcry, the Redmond, Wash.-based tech giant opted to let the other browsers back into MSN. However, it also took the opportunity to disparage the browsers' adherence to
industry standards. Microsoft warned, "the experience may be slightly degraded, simply because (those browsers) don't support the standards we support closely, as far as the HTML standard in those browsers" is concerned.

Opera is now accusing its rival of making sure that the MSN experience is degraded for Opera users.

Opera said that the page and style sheet served to IE 6 renders properly not only in that browser, but also in Opera 7. In addition, the code served to Opera has serious rendering problems in both.

"We know we have to bend over backwards to get Microsoft sites to work," said Jon von Tetzchner, chief executive of Opera, in an interview. "But it becomes extremely difficult when we are fed pages that don't work in any browser."

MSN's 2001 browser lockout provided ammunition for critics, including anti-Microsoft industry group ProComp, which accused Microsoft of unfairly exploiting its massive lead in the browser market to muscle out smaller competitors.

Opera, based in Oslo, Norway, said Microsoft's MSN maneuvers had already hurt its reputation.

One standards advocate dismissed the tiff between Microsoft and Opera as a sideshow to the more important issue of standard compliance.

"MSN is still living in a 1998 world of browser-specific workarounds," said Steve Champeon, chief technology officer of Hesketh.com and a
steering committee member of the Web Standards Project, an advocacy group.

"Whether Microsoft is intentionally trying to wreck Opera or not is really not the issue. The issue is their reliance on an outdated model, in which support costs scale exponentially and for which Microsoft itself is, to some extent, to blame. And that's the model we've been trying to move beyond for the past five years."